1. Field of the Invention
The present invention relates to a positioning control method, device for positioning the head of a disk device to a target position and a disk device, and more particularly to a positioning control method, device for detecting a head position from a position signal of a disk and a disk device.
2. Description of the Related Art
A disk device which reads a disk storage medium by a head is widely used. For example, a magnetic disk drive which is used as a memory of a computer comprises a magnetic disk, a spindle motor which rotates the magnetic disk, a magnetic head which reads/writes the magnetic disk, and a VCM actuator which positions the magnetic head to the track of the magnetic disk. The recording density of disk drives is dramatically increasing, and the track density of magnetic disks is also increasing. In particular, the use of an MR head for the magnetic head makes it possible to increase density. As a result, high-speed positioning with high accuracy is necessary.
FIG. 25 is a diagram depicting a position signal to describe a prior art, FIG. 26 is a diagram depicting PosN and PosQ thereof, and FIG. 27 is a diagram depicting the relationship between a demodulation position and a real position.
When a recording/reproducing command is received from a computer, the disk drive moves the magnetic head from the current position to the target position. This is called a “seek operation”. The seek operation is to shift to the follow control after the coarse control and the settling control. In order to control the magnetic head to the target position and to control the follow at the target position, feedback control is used. Feedback control detects the current position of the head, calculates the positional errors between the target position and the current position, and controls the actuator of the head so that the positional errors are zero.
To detect the current position of the head, a position signal shown in FIG. 25 is recorded on the disk. A position signal consists of a track number and an offset signal. The track number digitally indicates the track position. The offset signal is set to detect the offset position from the track center. In FIG. 25, for example, the offset signal consists of burst servo patterns PosA, PosB, PosC and PosD, which are four phases with a 90 phase shift respectively.
The head reads the position signal of the disk, and the current position is demodulated from the read position signal. The position signal read by the magnetic head is demodulated by the demodulation circuit, and the track number and the offset signal are obtained. The offset signal is obtained from the amplitude of the above mentioned servo signals, and the magnitude is in proportion to the positional deviation from the track center of the track number.
The position signals with two phases, PosN and PosQ, shown in FIG. 26, are demodulated from the amplitude of the servo patterns in FIG. 25. These position signals have a 90 phase deviation. For example, the position signals PosN and PosQ are calculated by the following formulas (1) and (2).PosN=PosA−PosB  (1)PosQ=PosC−PosD  (2)
The demodulation position (current position) is demodulated using the linear part of the position signal. This demodulation position is obtained by calculation. For example, the demodulation position (Position) is calculated by the following formula (see for example, Japanese Patent Laid-Open No. H8-195044). In other words, the magnitude of the absolute value abs (PosN) of PosN and the magnitude of the absolute value abs (PosQ) of PosQ are compared, and if abs (PosN)≦abs (PosQ), the following demodulation position is obtained by the following formula (3).Position=−sgn (PosQ)*PosN+Track  (3)
If sgn (PosQ)*even (Track)>0.0, however, the following formula (4) is added to the formula (3).Position+=sgn (PosQ)* sign (PosN)*1.0  (4)
If abs (PosN) is not≦abs (PosQ), on the contrary, the following formula (5) is used.Position=sgn (PosN)*(PosQ+even (Track)*0.5)+Track  (5)
Here sgn ( ) is a sign of ( ), Track is a track number, and even (Track) is “1” if the track number is an even number, and is “0” if the track number is an odd number. The C language program describes this as follows.
if (abs (PosN)≦abs (PosQ)){
Position=−sng (PosQ)*PosN+Track;
if (sgn (PosQ)*even (Track)>0.0)
Position+=sgn (PosQ)*sgn (PosN)*1.0;
} else {
Position=sgn (PosN)*(PosQ+even (PosQ+even (Track)*0.5)+Track;
}
FIG. 26 shows PosN and PosQ at this time, and FIG. 27 shows the relationship between the demodulation position obtained by calculation and the real position.
The conventional demodulation method, however, sets such that an accurate position is obtained when the speed of the head, that is, the speed of the actuator, is zero. In other words, the conventional demodulation method is for obtaining an accurate demodulation position during the above mentioned follow where accuracy is not considered when the speed is not zero. Conventionally, it was known that the head crosses the servo pattern diagonally when the speed of the head is not zero, as shown by the arrow marks in FIG. 26, but the accuracy of the demodulation position in this case is not considered.
Along with the recent demand for higher densities and higher speeds of a disk drive, the accuracy of the demodulation position is also demanded during seek control, that is, when the speed is not zero. For example, for the above mentioned settling control, the speed control is switched to the settling control when the head reaches before several tracks before the target track. Reaching the several tracks before the target track is detected from the positional errors between the target position and the demodulation position. That is, this detection result is obtained from the demodulation position when the speed is not zero.
If this demodulation position has errors, the track position just before shifting to the settling control changes, so the time required for the settling control changes. Since high speed is now demanded, errors in the time required for the settling control causes a fluctuation in the seek time. Especially when the target speed of the speed control is set higher to increase the speed of the seek time, the head speed at the start of settling control becomes faster, and errors in the demodulation position further increase, which interferes with the increasing speed of the seek time.
Also for the speed control, the target speed is generated from the abovementioned positional errors, so if the demodulation position has errors, the target speed also differs from the predetermined speed, and the seek performance expected in design cannot be obtained.